135 research outputs found
Dynamical Objectivity in Quantum Brownian Motion
Classical objectivity as a property of quantum states---a view proposed to
explain the observer-independent character of our world from quantum theory, is
an important step in bridging the quantum-classical gap. It was recently
derived in terms of spectrum broadcast structures for small objects embedded in
noisy photon-like environments. However, two fundamental problems have arisen:
a description of objective motion and applicability to other types of
environments. Here we derive an example of objective states of motion in
quantum mechanics by showing a formation of dynamical spectrum broadcast
structures in the celebrated, realistic model of decoherence---Quantum Brownian
Motion. We do it for realistic, thermal environments and show their
noise-robustness. This opens a potentially new method of studying
quantum-to-classical transition.Comment: 6 pages, 3 figures, accepted for publication in EP
Spin squeezing inequalities and entanglement of qubit states
We derive spin squeezing inequalities that generalize the concept of the spin
squeezing parameter and provide necessary and sufficient conditions for genuine
2-, or 3- qubit entanglement for symmetric states, and sufficient condition for
general states of qubits. Our inequalities have a clear physical
interpretation as entanglement witnesses, can be relatively easy measured, and
are given by complex, but {\it elementary} expressions.Comment: formula (24) corrected, minor changes, final versio
Quantum superadditivity in linear optics networks: sending bits via multiple access Gaussian channels
We study classical capacity regions of quantum Gaussian multiple access
channels (MAC). In classical variants of such channels, whilst some capacity
superadditivity-type effects such as the so called {\it water filling effect}
may be achieved, a fundamental classical additivity law can still be
identified, {\it viz.} adding resources to one sender is never advantageous to
other senders in sending their respective information to the receiver. Here, we
show that quantum resources allows violation of this law, by providing two
illustrative schemes of experimentally feasible Gaussian MACs.Comment: 4 pages, 2 figure
Gaussian work extraction from random Gaussian states is nearly impossible
Quantum thermodynamics can be naturally phrased as a theory of quantum state
transformation and energy exchange for small-scale quantum systems undergoing
thermodynamical processes, thereby making the resource theoretical approach
very well suited. A key resource in thermodynamics is the extractable work,
forming the backbone of thermal engines. Therefore it is of interest to
characterize quantum states based on their ability to serve as a source of
work. From a near-term perspective, quantum optical setups turn out to be ideal
test beds for quantum thermodynamics; so it is important to assess work
extraction from quantum optical states. Here, we show that Gaussian states are
typically useless for Gaussian work extraction. More specifically, by
exploiting the ``concentration of measure'' phenomenon, we prove that the
probability that the Gaussian extractable work from a zero-mean energy-bounded
multimode random Gaussian state is nonzero is exponentially small. This result
can be thought of as an -no-go theorem for work extraction from
Gaussian states under Gaussian unitaries, thereby revealing a fundamental
limitation on the quantum thermodynamical usefulness of Gaussian components.Comment: 7+8 pages, 2 figures, close to the published versio
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